1. Field of the Invention
The present invention relates to a liquid crystal image display device and a method of manufacturing the same. More particularly, the present invention relates to an image display device which provides an improved image display and a method of manufacturing the same.
2. Description of the Related Art
In recent years, liquid crystal image display devices have been attracting attention as providing thin and lightweight devices intended to replace cathode ray tube image displays. A cross-sectional view of a cell portion of a conventional liquid crystal image display device is shown in FIG. 3 in which reference numeral 100 denotes a driving transistor portion for driving a liquid crystal; 101 denotes a glass substrate; 102, 103 and 104 respectively denote a source portion, a drain portion and a gate portion of the driving transistor portion 100, a signal voltage to be applied to a liquid crystal layer being sent to a transparent electrode 106 from an interconnection 105 through the driving transistor portion 100; 107 and 109 denote an insulating layer; 108 denotes a liquid crystal layer in which a liquid crystal is driven by a voltage applied between a transparent electrode 110 and the transparent electrode 106; and 111 denotes a glass substrate on which a light blocking layer 112 is formed to prevent light from striking the driving transistor portion 100.
The typical liquid crystal cell in FIG. 1 has drawbacks in that (1) there are variations in the liquid crystal cells as well as an unstable alignment of the liquid crystal, and (2) there is light leakage from the adjacent individual pixels which can cause cross-talk. These drawbacks are explained in more detail below.
(1) Variations in the liquid crystal cells and unstable alignment of the liquid crystal
As can be seen from FIG. 1, the liquid crystal layer 108 is provided on the driving transistor portion 100 and the interconnection layer 105. Both of the driving transistor portion 100 and the interconnection layer 105 define "shoulders" where the thickness of the liquid crystal layer 108 between the transparent electrodes 106 and 110 serving as the display electrodes varies in the cell, as indicated by intervals 113, 114 and 115 in FIG. 1. This makes impossible applying a uniform electric field to the liquid crystal layer 108. In a color display device, for example, the alignment of the liquid crystal changes slightly and thus generates an undesirable change in the color. Furthermore, due to problems related to the production process, there are variations in the shoulders of individual cells such that the display characteristics of the individual pixels differ, generating a fixed pattern noise.
These shoulders cause problems even in a display device with pixel dimensions as small as 60.times.89 .mu.m. To overcome this deficiency, the effective portion may be provided by a flat area alone. However, this reduces the aperture ratio and thus darkens the display screen. Furthermore, in a desirable display device in which the number of pixels has been increased from several ten thousands used currently to a range of from several hundred thousands to one million, the pixel size is further reduced to a range of from 20.times.30 .mu.m.sup.2 to several tens .mu.m.sup.2, practically eliminating the flat area. The shoulders may be reduced also by increasing the thickness of the insulating layers 107 and 109. However, this weakens the electric field effectively applied to the liquid crystal layer, and thus necessitates a corresponding increase in the driving voltage. Since the liquid crystal driving voltage is some 5 v higher than that applied to a normal semiconductor device, any further increase is disadvantageous in terms of the maximum voltage that the driving transistor can withstand. Thus, the reduction in the deleterious effect of the shoulders cannot easily be attained in practice.
Since the thickness of the liquid crystal layer 108 is particularly small in a ferroelectric liquid crystal display device, even a slight change in the thickness of the ferroelectric liquid crystal layer may greatly affect the display characteristics, making high-definition display difficult.
Furthermore, in a display device in which the electrical capacitance of the liquid crystal (the "liquid crystal capacitance") is not sufficiently large or in which the effective resistance parallel-connected to the liquid crystal capacitance is low, a storage capacitance must be parallel-connected to the liquid crystal capacitance. However, the deflected voltage may also be reduced by increasing the storage capacitance parallel-connected to the liquid crystal capacitance, reducing the aperture ratio and degrading the reliability of the insulating film which forms the storage capacitance.
In the aforementioned conventional structure shown in FIG. 1, since the interconnection layer connected to the source 103 and gate 104 of the thin-film transistor is present on the same surface as that of the display electrode 16, the capacitive coupling between the interconnection and the display electrode is large, thus causing deflection of the display electrode potential due to cross-talk and deteriorating the display quality of the liquid crystal display device. To reduce cross-talk, the distance between the interconnection layer and the display electrode may be increased. However, this reduces the aperture ratio. Reduction in cross-talk may also be achieved by increasing the thickness of the interlayer film provided between the interconnection layer and the display electrode. However, this increases the magnitude of the shoulders.
Additionally, the liquid crystal molecules are normally aligned to the substrate layer by rubbing. Rubbing is performed by mechanically rubbing the surface of the substrate. However, it is difficult to uniformly rub the entire area of the substrate having an interconnection providing a large number of shoulders. Thus, the liquid crystal molecules may not be aligned uniformly, causing a display failure or a difference in the characteristics.
(2) Light leakage from among the individual pixels
As shown in FIG. 1, the light-blocking layer 112 is formed separately from the driving transistor portion 100. Consequently, when a light enters the liquid crystal cell from above as viewed in FIG. 1, oblique or diffracted light may enter adjacent pixels, generating cross-talk. Furthermore, the light illuminated on the driving transistor portion 100 generates unnecessary carriers in the driving transistor portion 100, resulting an occasional malfunction. Therefore, in order to eliminate the problem involving light leakage, the size of the light-blocking layer is increased. This greatly reduces the effective aperture ratio, as in the case of the aforementioned problem (1).
Furthermore, where the light-blocking layer 112 is provided, it is necessary to provide both the light-blocking layer and an insulating layer, in addition to the interconnection layer. This further increases the magnitude of the shoulders.